Pigment manufacture



Patented Dec. 29, 1936 Arthur Minard Brooks and Harold Robert Rafton, Andover, Mass, assignors to Raifold Process Corporation, a corporation/of Massachusetts No Drawing, Application August 10, 1933, Serial No. 684,604 q 27 Claims. (Cl. 13458) This invention relates to the manufacture of pigments in finely divided form.

The principal object of this invention is to pro duce a pigment in finely divided form conjointly with the production of caustic alkali in the causticizing process.

. An important object of the invention is to produce a finely divided pigment in the causticizing operation by controlling the temperature of the .10 reaction, and by employing preslaked lime con-' I taining magnesia.

A further object'bf the invention is the production of a finely divided pigment in the causticizing operation by controlling the conditions of agitation.

A further object of the invention is the production in soda or sulphate pulp mills of a finely divided pigment, useful for filling or coating paper.

will become apparent during the course of the following description. In our copending application Serial No. 628,616, filed August 12, 1932, of which the present ap- 2 plication is a continuation in part, and to which cross reference is hereby made, we have described the production of a much more finely divided causticizing sludge than is usual, by conducting at least a substantial part of the causticizing reaction with preslaked causticizing agent below 75 C.', and preferably not higher than 70 C. While that application concerns causticizing agents in general, and more particularly relates to high calcium lime, it is stated therein that when a similar procedure is undertaken with lime containing magnesia, for example dolomitic lime, an analogous enhancement of fineness of particle size re-- suits. No claims are made specifically tov lime containing magnesia in our above mentioned c0- 40 pendingapplication, and it is the purpose of the present application to present such claims.

In Raftons Patent No. 1,415,391, issued May 9,

1922, there is disclosed a white pigment and a method of producing the same. The pigment has been found to consist substantially of calcium carbonate magnesium hydroxide in a fine state of subdivision. The method of manufacture of the pigment disclosed therein comprises slaking lime containing magnesia, adding sodium carbonate, agitating and boiling, whereupon the pigment above described is produced in conjunc tion with sodium hydroxide. v e

We have found that if instead of conducting the above reaction at substantially boiling, it be conducted at or below 75 C., and preferably not Other objects and advantages of the invention higher than 0., the fineness of the particle size of the precipitate is substantially enhanced and under certain conditions the precipitate may approach or be in the colloidal condition.

Briefly described, the present invention involves 5 subjecting sodium carbonate to the action of previously slaked lime containing magnesia in an aqueous medium at a temperature of or below 0., preferably of or below 70 0., preferably under conditions of vigorous agitation. The fol- 1o 'lowing steps may conveniently be taken in carry-' ing'out the invention; slake the lime containing .magnesia thoroughly in water; if necessary, ad-

just the" temperature of the slaked lime to the desired point; '-mix the slaked lime with sodium 15 carbonate which may conveniently be in solution at the.desired temperature; agitate the mix at the desired temperature or in the desired temperature range; separate the liquor from theprecipitate as by filtration; and wash the precipitate to 20 separate soda salts more or less completely as desired. If for any reason it be desired to have the precipitate retain its free lime content, if any, or to have it converted by any method other than the use of sodium carbonate, as for instance by acid, then the soda salts may be removed as completely as feasible. However, if sodium carbonate isto be used for substantially removing any free lime present, then the precipitate is preferably treated with sodium carbonate in an aqueous medium, the liquor separated and the precipitate washed. In order to remove any particles of oversize which may be present, any suitable procedure -maybe employed, such as screening, which may be carried out at any time as in the presence of 35 liquor or water. The precipitate may be used directly or dried before use or otherwise handled or treated in any known manner. The causticized soda liquors may becombined if desired for any specific purpose, for exampleforthe cooking 40 of wood by the soda pulp process orthe production of caustic soda for the chemical market.

As wilLbe apparentthe several steps taken in carrying out the invention are susceptible to yariations in their practice and these steps, and the preferred methods of carrying them out, together with variables influencing them will be discussed in detail below. However, in order that this invention may be fully understood there is given first a, concrete illustrative example of one for- 50 mula and procedure which is satisfactory for the carrying out of the invention in practice, but which is to be considered in no way as limiting.

5000 lbs. of dolomitic lime are slaked in 1800 gallons of boiling water in an u right cylin tank 9 feet in diameter and 9 feet high, equipped with a strong agitating device comprising ahorizontal arm attached to a vertical shaft, the arm being located about 2 feet above the tank botical point.

tom and being equipped with flexible steel fingers whereby the lime may be thoroughly stirred during slaking and may finally exist in the form of a thick paste or mud. The thoroughly slaked mass is then allowed to cool down to a temperature of approximately 50 C. There is then added in solution 4800 lbs. of sodium carbonate, at a as a high speed agitator or the like, and the mixing continued from 2 to 3 hours. "The batch is then heated substantially to boiling, allowed to run at about that temperature for about /2 hour, filtered on a filter press to separate the liquor from the mud, and the bud further treated with approximately 1000 lbs. of sodium carbonate in solution at a temperature at or near boiling for about an hour, the total volume of the mix being 3800 gallons, and then a second filtration made, with thorough washing of the mud. The liquors from the first and second filtration may be so combined as to yield a liquor of approximately per cent. causticity at about 11 to 11 B.-which is suitable for use in cooking wood by the soda pulp process. The mud produced will be found to be of extremely fine grain, and after passing through a. suitable screen to remove oversize may be used, for example, as a filler for paper and also, preferably after suitably dewatering, as a, coating for paper.

The invention disclosed herein is based on the discovery that when employing previously slakedlime containing magnesia in the causticizingreaction the temperature of causticization infiuences profoundly the particle size of the resulting precipitate so that by properly controlling the temperature below a certain critical point a precipitate of substantially finer particle size may be produced than if the reaction were carried out at a higher temperature above the crit- The temperature range employed for conducting the causticizing operation in accordance with the present invention is that from 75 C. down to room temperature or lower, even down to the freezing point of the mix. As is well recognized, limes vary greatly one from another, but in spite of these variations it has been found that most limes containing magnesia so far tested show a marked enhancement in the fineness of particle size of the precipitate producible therefrom when employed in the causticizingreaction (with previous slaking) at a temperature of 75 C. or below,irom what they show at any temperature ranging from above 75 C. to that of boiling. Some limes, however, have been tested which exhibit the dividing point between fine and much finer graimprecipitateaprodueed at a temperature of 70 C. Thus, the upper point in temperature which may be used in the present invention is seen to vary somewhat with the lime employed, but considering the wide-variati0h in. 1 1 1 5 50 far tested, this upper temperature point, or critical temperature, seems to be substantially def-' inite.

In determining this point, no attempt was made to make successive tests closer than at 5 intervals; tests were made, for example, at temperatures from boiling to room temperature and below, so that when the statement is made herein relative to the critical temperature at which a.

very fine particle size precipitate is produced, it means that the test conducted at the next 5 interval above that point did not show so fine a particle'size whereas that particular test did show very fine particle size. 7

The dividing line between very fine and fine grain precipitates is, as stated, quite sharp but the fineness of the precipitate seems to be still more enhanced in most cases at a reaction temperature of 60 C., and to be even finer at a reaction temperature of approximately 50 C. Below that point in some cases the precipitate produced seems to be somewhat finer than that produced at 50 C. and in some cases not, whereas with certain limits an intermediate point between 50 C. and very low temperatures appears to be the temperature of maximum fineness. In some cases a temperature or approximately 20 C. gives a very fine or even colloidal precipitate. Thus, there seems to be no general statement that can be made covering the tendencies of all the limes tested below 50 C., but the statement can be made that at reaction temperatures of 75 C. or below with most limes and of 70 C. or below with others, the particle size of the precipitates produced is distinctly finer than that of those produced above those temperatures, that at 60 C.,

the particle size is finer-than at 70" C., and that v in most cases the particle size at 50 C. or below mix to cool to room temperature as the reaction progresses. Other suitable methods are feasible,

for example, such as those mentioned elsewhere herein; A peculiar thickening or gelling has been found to occur at times at lower temperature ranges and a discussion of this and methods of avoidance thereof, if desired, is given later.

Relative to time requirements, it has been found that at 50 C., using concentrations which produceliquors suitable for wood cooking, suitable causticity may be reached in approximately 4 to 8 hours, and it may be stated that the time required for carrying out the reaction is in general greater the lower the temperature. However, although in certain cases it may be desired to maintain the specified temperature or temperature range throughout the mixing or mixings, the important point is the conducting of a substantial if not the major part of the reaction initially at the temperature specified and thus after this has been accomplished, where the time factor is important, the temperature may be subsequently raised even to boiling to bring about the conversion of the sodium carbonate much more speedily, and permit a more speedy filtration due to decrease of viscosity of the solution with rise in temperature. Thus, a preliminary mix at the desired temperature for 2 "to 3 hours, followed by boiling, reduces the time of the material when subsequent high tempera-' tures are used should be checked against batches run wholly within the specified temperature range to be sure that the danger point in such practice has not been reached. The times required may-be shortened in other ways also, such as by increasing the degree of agitation, increasing the proportion of sodium carbonate to lime and, if permissible, from the standpoint of ultimate use of liquors, by decreasing the concentration of the mix.

Relative to agitation, this has been found to have a very important bearing in certain cases on the quality of the precipitate produced. The preferred method of carrying out the invention is to perform the causticizing reaction under conditions of vigorous agitation.

Where the lime is slaked to a thick slurry with the agitator described above, such agitator, necessarily running at slow speed, gives a degree of agitation in the tank, after the mix has been made up with the sodium carbonate solution, which may not be adequate. Providing there is no way of increasing the speed of such an agitator, it is preferable after the few minutes required to preliminarily incorporate the slaked lime slurry with the solution, to transfer the mix to a second tank equipped for vigorous agitation so that the reaction may be conducted under conditions of vigorous agitation. There are a number of well known agitating devices on the market which give satisfactory results, and although it is not the intention of limiting the carrying out of the reaction to any one type of apparatus, it can be stated that an efiicient type of agitator for the purpose is a high speed propeller operating in a. draft tube centrally located in a mixing tank, to insure vigorous agitation accompanied by efficient circulation. Agitation with air when properly applied may be used if desired, particularly as auxiliary agitation.

Regarding lime, we employ lime containing "magnesia to produce the pigment comprising substantially calcium carbonate magnesium hydroxide. Of course it will be understood that by using limes of varying magnesia contents, the quantitative composition of the pigment produced will be changed, but the pigment will still retain its same general qualitative character.

While we do not limit ourselves to limes containing any definite magnesia content, because we have found limes of widely varying magnesia content suitable for our purpose, nevertheless we find that dolomitic lime, that is lime containing the calcium and magnesium radicles in substantially equimolecular proportions, gives excellent results, and as this lime is widely distributedand procurable at a relatively low cost, this is a convenient lime containing magnesia for our use.

Limes may be used which are made from magnesia-containing limestones from various geological formations, as well as reburned mag: nesia-containing lime, L kewise limes of varioustypes such as lump, pebble and granular may be used and also limes produced in various ways, for example, in upright kilns, rotary kilns, sintering machines and the like. In general the lime used should preferably possess the followin properties: it should slake well, beof good white color, be as. free as feasible from unburned cores, and have a low silica content, When high calcium lime is slaked, the slaked lime has the composition of calcium hydroxide. When lime containing magnesia is slaked, the slaked lime indicates on analysis the empirical composition of calcium hydroxide magnesium hydroxide. When this slaked lime is used in the causticizing process, the magnesium hydroxide is substantially unaffected therein, and the final reaction product consists substantially of calcium carbonate magnesium hydroxide, as disclosed in Raftons patent above referred to.

While the lime may be slaked in any desired amount of water, it-is preferable to slake it in such an amount as to yield a thick mud or slurry. Although not the preferred method, the slaking may be conducted in liquors containing a relatively small amount of sodium carbonate, as for example in weak wash liquors from the process, whereby a somewhat inferior product is obtained. However, the lime must not be slaked in the main body of the liquor containing the sodium carbonate to be causticized, as under such conditions only relatively coarse grain precipitates are produced regardless of reaction temperature. The slaking water may be cold, boiling or at any intermediate temperature. In general, however, the best results appear to be more easily obtainable using lump quick lime and slaking in boiling water and this is our preferred method. While limes vary considerably in the amount of slaking water necessary to convert them into a thick slurry, nevertheless the proportions given in the illustrative example above will be found to be about the average required with lime of good grade. Sufilcient time should be allowed for complete slaking to bev effected. While a shorter time may be used, it is better to. allow at least 1 to 2 hours to be sure of getting good results, and with certain limes even longer slaking times have been found advantageous.

'As has been indicated above, the final precipitated product may'be processed, for example to free it from oversize by any method desired such as flotation, sieving, grinding or the like. It is also apparent that if desired the lime used in the reaction may be subjected to similar processing, preferably after the slaking has been completed. Such operation may be conducted on the lime in addition to that conducted on the final product,or in lieu thereof, but it is' usually preferable to conduct it on the final product; The reason for this is that in order to sieve slacked lime satisfactorily it should be in relatively dilute suspension, and when concentrated caustic liquor is desired to be produced, such dilute lime suspension ordinarily requires concentration prior to its use in the causticizing reaction, such as by filtering or the like, and as this introduces an additional step, it is usually more desirable as stated above to confine such refining operation to the calcium carbonatemagnesium hydroxide produced rather than performing it in connection with the lime used.

One type of previously slaked lime containing magnesia is what is known in the trade as hydrated lime. Such lime is made by reacting upon lime with water in suflicient quantity so that the final product while containing substantially no calcium oxide will nevertheless be substantially dry to the touch. Hydrated'limes are usually made'in some type of commercial hydrator, usually continuous, by feeding in lime and the desired quantity of water, whereupon hydrated lime is produced which may be ground, sieved, or air floated if desired. Qr hydrated lime' may be prepared directly with somewhat more water to produce instead of a completely dry material a material which may have a certain amount of water in it.

It has found that while hydrated limes containing magnesia may be used in the present process, the results obtained therewith are usually substantially inferior and in most cases very much inferior, to those obtained when using lime preslaked in an excess of water. It has been found that in addition to not obtaining in general so fine a particle size precipitate with the hydrated limes, there usually exists a large proportion of grit and oversize in the final product made therefrom, which proportion is usually much greater than when employing lime preslaked with a substantial excess of water. Moreover, it has been found that with the hydrated limes so far tried, such limes seem to be effective even in their limited capacity to produce very fine precipitates,

only substantially at the lower temperatures, as they do not appear to give any substantial improvement in fineness at or near the critical temperature. Thus, while in certain cases hydrated lime or hydrated lime containing some moisture may be employed, such lime is not preferred, owing to the inferior product generally produced thereby.

Relative to the sodium carbonate employed,

this ma be solid soda ash, (if there is provided sufi'lcient water to dissolve it) or a. solution thereof, or any material or solution containing sodium carbonate, such as reclaimed soda recoveredfrom sperlt liquors in the manufacture of pulp. There may be present, also, non-causticizable or partially oausticizable constituents in solution, withoutapparent detriment to. theprocess. Examples of solutions containing such, are solutions I such as are causticized in sulphate mills, Keebra process liquors, or the like. 7 I i The solutions used preferably should be filtered, settled or the like before use if any solid impurities are present. Carbon particles and the like such as may occur in the liquors to be causticized in soda and sulphate pulp mills if not removed, may show up later as black specks or discoloration in the precipitate. The eil'ectof heat of solution of solid sodium carbonate, ii used, should be taken into consideration when adjusting the reaction temperature.

While the commercial importance or this invention resides chiefly in the use of lime containing magnesia as causticizing agent and sodium carbonate as causticizable salt, other causticizable or partially causticisable salt may be used, to produce in general a soluble alkaline hydroxide and a substantially insoluble precipitate which may consist substantially of magnesium hydroxids and a calcium compound of chemical composition corresponding to the acid radicle of the salt or salts used. Examples of causticizable or partially causticizable salts are such salts as may be present in or derived from liquors used for digestion or processing in pulp manufacture, and those or other salts when present in substantially pure condition, such as fluorides or sulphates. Normal salts may be used in the present invention or acid salts, such as sodium or potassium bicarbonate, bisulphate and the like.

Relative to proportion of ingredients, the proportion ofline containing magnesia to soda ash should be chosen to give liquors of the desired causticity and preferably to give a precipitate to give the desired final liquor.

from residual calcium hydroxide and a blended.

final caustic liquor of substantially 90 per cent. causticity. These proportions are of course merely illustrative as others may be chosen as desired. In some cases a large excess of lime may be used in the first mixing in order to get liquors of high causticity in a short time, and then the carbonation of the residual calcium hydroxide content of the precipitate may be completed in the second mixing by using the proper amount of sodium carbonate. On the other hand, larger amounts of sodium carbonate may be used, in only one mixing if desired, if the resultant lower causticity liquor can be utilized, or converted to a higher causticity liquor by the action of further quantity of lime.

In preparing the slaked lime containing magnesia and sodium carbonate solution for use, both may be adjusted to the desired temperature before mixing or the temperatures may be difierent providing that, upon mixing, the batch will be at the desired temperature. However, in general the former method is preferred. The apparatus used may be insulated, and equipped with thermostatic control if desired so that the temperaturemay be'maintained at the required point either by admitting steam directly into the re-,

action vessel or steam or cold water into a jacket surrounding the same, when one is provided.v

' final concentrations. For example, using the illustrative proportions given in the illustrative example above, the volume in each of the two mixings may be 3,800 gallons as stated. This is equivalent to approximately .7 lb. of calcium oxide present in the lime originally used per gallon of total volume and produces a liquor in the first mix, ofabout 16 36., which may be combined with the liquor from the second mix, to give a total combined liquor of approximately ll-11V Be. at about 90 per cent. causticity as stated. Using the same proportion of ingredients, but carrying out the first mix at a lime concentration of about .47 lb. per gallon of total volume, the liquor from the first mix may be about 9 Be. and 94 per cent. causticity.

The concentration employed appears to have an important bearing on the particle size of the precipitate produced. At the higher concentrations, such as .7 1b., of lime or more per gallon of total reaction mix there is produced in the pres-. ent invention, as stated, a very finely divided precipitate. When, however, the reaction is carried out at lesser concentrations, such for example as .47 or .18 lb. of lime per gallon, in certain cases the precipitate produced seems to be of such finely divided nature as to possess gelatinous characteristics, and appears to be of acolloidal nature. of especial value for certain purposes, as is de- Precipitates of such nature are scribed in detail below. When in the'specification wespeak of the concentration of lime in a mix as being a given number of lbs. per gallon, we mean that the calcium oxide content of the mix equivalent to the calcium compound content thereof is present. in such number of lbs., and not the combined equivalent calcium and magnesium oxides content.

' It will be apparent that the mix or mixings may be carried out at superor sub-atmospheric pressure but owing to the greater simplicity of the apparatus involved it is preferred to operate at atmospheric pressure.

As briefly referred to above, when the initial temperature of the mix is in the lower ranges there usually appears a transitory thickening of the batch within a very few minutes, which,

unless'the mix sets to a solid gel, disappears upon continued agitation and does not reappear. This is particularly so when sodium bicarbonate is present as a constituent. At the higher temperatures this efiect is not so prominent. When the concentration of the batch is approximately .7

. lb. or more of lime per gallon of total volume,in

many instances 'the mass sets to a solid gel in a' few minutes when-temperatures below approximately 40 C. are employed. The gel may be so firm as to allow agitation only in the small area actually out out by the agitator. This may be particularly thecase if some bicarbonate be present in the sodium carbonate used. Inthe event of such gel formation it is desirable to employ an auxiliary agitator or device to scrape the sides of the reaction vessel in order to break down the gel, which however, may be readily accomplished. After the mix in the gel form is broken down to a slurry, it does not apparently set again into the gel form even though the reaction is continued to completion at the same temperature and at the same rate of agitation as that at which the-gel originally formed. As it is .sometimes inconvenient to operate with gel formation, a. number of methods of'overcoming the thickening to a solid gel have been devised, of which several will be mentioned below.

One of these is by the regulation of the temperature. Without changing concentration, agitation', or proportion of ingredients, but 'by operat-- ing above approximately 40 C., solid gel formation is not encountered although a transitory thickening of fire batch may be noted. .Also by adjusting the initial temperature just above. the range where gelling takes place, for instance at 40-50? C.,. the batch may then be allowed to cool down to room temperature-approximately C., and in this manner the initial setti g of the batch is overcome. As stated above, thickening appears tobe an initial phenomenon and once the period in whichit is likely to occur has passed it need be given no further consider- I ation in adjusting the conditions of the operation.

Without changing the concentration, the temperature or agitation, the lime may be added in stages instead of all at once to overcome setting to a solid gel, but with somewhat lesser efiiciency as regards removal of soda salts from the precipitate. in the first mixing. Thus, at the start of the reaction only half the slaked lime may be added and all the sodium salt, with later addition of the remainder of the slaked lime. The precipitate is of very fine particle size in general substantially characteristic of that producible at the same temperature were all the ingredients to be .part of the sodium salt, does not appear to be useful in preventing gel formation where it would otherwise occur; on the contrary, it appears to intensify the gelling phenomenon.

' Without changing the concentration, temperature or method of addition of ingredients, setting to a solid gel may alsobe overcome in certain casesby regulating the agitation of the batch carefully, e. g. by mixing the ingredients initially for one or two minutes, then stopping the agitator for a period of to 4 hour and'then resuming the agitation. During this period of non-agitation there may be a transitory thickening of the batch but it does not-set to a solid gel; Here again, although the quality of the precipitate is satisfactory, the soda salts, however, are apparently substantially removed only after extensive washing.

By using slower agitation at the start, e.'g. just suflicient to keep the solids in suspension, a solid gel will not form in certain cases, but clots of gelatinous material may appear which upon prolonged mixing will disintegrate. However, in this case there is some decrease in the quality of the precipitate, since the particle size is apparently not so exceedingly fine as that producible with more vigorous agitation.

Without changing the temperature or agitation or using step addition of lime, setting to a solid gel may be overcome by changing the concentration. Thus, instead of operating at aconcentration of about .7 pound lime per gallon of total volume, if the operation is conducted at a lower concentration as for example .47 or .18 lb. lime per gallon of total volume and at 20 0., no gelling of the mix is usually encountered. There may appear a transitory thickening which, however, does not result in a solid gel.

Also, apparently in certain cases the gel may be reduced to liquid condition by heating up the gelled mix, say to a temperature of C., preferably with some agitation. v

It is characteristic of the precipitates which are produced in the present invention that bethat the equipment would thus become so costly.

that filter pressing would be indicated from the standpoint of economy. The washing of the precipitate and routing of liquors may be handled countercurrently in known manner. Thewhole operation may likewise be carried outin continuous manner while observing the essential features of the process. The precipitate may be used in pulp condition as produced or in'liquid suspension, or dried for use as desired.

In certain cases where it is desired to produce the very finely divided precipitate of calcium carbonate'magnesium hydroxide, but no use exists for the caustic soda concurrently produced, the liquor from the process may be carbonated, as by.

carbonic acid gas,'and reused cyclically in the process.

In certain cases where it is desired to obtain a precipitate of a gelatinous or colloidal type, the production of which is in many cases promoted by dilution and/or low temperature, but where dilute caustic liquors which may be produced therewith are unacceptable, the e'fiect of dilution in the reaction may be, with a measure of success, obtained by carrying out the reaction dilute in respect to the lime present rather than in respect both to the total soda salts and lime. ample, a solution containing considerable caustic soda but with only substantially sufilcient sodium carbonate to react with a relatively small quantity of lime may be employed to react with such small quantity of lime whereby the concentration of such lime and sodium carbonate in respect to the total volume is relatively low. In certain cases this reaction may be carried out in step fashion, for example by adding successive quantities of the sotiium carbonate to the already present lime, and/or may be carried out continuously if desired. These expedients and other similar ones which will readily occur to one skilled in the art. however, involve certain complications which the straight dilute liquor procedure avoids.

The precipitates of the present invention may be used in the manufacture of filled paper with excellentresults. The type of precipitate used may be chosen according to the character or the paper desired. Thus, where the paper is to have a very high finish the gelatinous precipitates may be used but where not so high a finish is wanted,

the non-gelatinous precipitates may be used withv satisfaction. In the manufacture of coated paper the diiferent precipitates 'may be used in order to obtain specific results. Thus, with the use of non-gelatinous precipitates, glossy coated paper may ordinarily be produced with the usual degree of calendering, while with the gelatinous precipitates much 'higher finishes may be ob-* tained.

It should be noted that in the past in order to use ordinary lime mud, i. e. calcium carbonate, advantageously as a pigment for coating .paper, it was necessary to grind it-beiore use as disclosed in Raftons Patent No. 1,260,448, issued- March 26, 1918. Likewise, such procedure has been found to enhance the quality of filled paper made with lime mud. However, in using the present precipitates no mechanical treatment'to reduce or affect the particle size is required before use. However, of course, even these very fine grain precipitates may be subjected to grinding if desired, but this has been found to be un-' necessary in practice.

There are a number 01' other uses for the precipitates herein produced but there has been indicated first the uses in the paper industry because in many'cases the precipitate maybe produced in a pulp mill in which pulp is produced by an alkaline process such as the soda or sulpounding ingredients in the rubber industry, and

use in the manufacture of color lakes, printing inks, etc. I

Although in general the chemical composition For exof the product when lime containing magnesia and sodium carbonate are used in the present modified causticizing reaction is considered to be calcium carbonate magnesium hydroxide, yet in many cases, as for example, where colloidal precipitates are produced, there may be some doubt as to the exact chemical and structural compo-, sition oi the precipitate and therefore where in the claims "calcium carbonate magnesium hydroxide is used there is meant to be included calcium carbonate magnesium hydroxide or any complex or compound such as would result in the present invention, comprising calcium, carbonate, magnesium, and hydroxyl radicles.

In respect to a reaction in the presence of water involving lime and a material other than a caus-' ticizable alkali metal salt, said material being reactive with said lime to produce substantially water insoluble calcium compound, said material being used in sufilcientquantity to efiect conversion of nearly all, i. e. approximately to or thereabout, or 01' all of. said lime into said substantially water insoluble calcium compound, it is to be understood that the claims are not. meant to include within their scope the practice of including in such reaction in the capacity of a catalyst, intermediate acting or exchange material, or the like, a causticizable alkali metal salt in small percentage, 1. e. 10% to 15% or thereabout on the weight of the CaO present in the lime employed, which ,salt may incidentally produce a substantially correspondingly small amount or less or alkali metal hydroxide by reaction with said lime.

Where in the claims the expression calculated in the form originally used" is employed as applying to the causticizing agent in defining a concentration, the weight meant to be used in the calculation oi saidconoentration is the weight oi. the calcium oxidein the causticizing agent prior to slaking.

The term causticizable salt as used in the specification and claims is intended to include those salts which are either wholly or partially .try and the like, to designate an alkali metal salt which will react in the presence of. water with lime, to produce an alkali metal hydroxide and a'substantially water'insolubie material, or at least one substantially insoluble under the conditions of the reaction, said'mate'rial being of a chemical composition corresponding to the acid radicle'oi the salt used. -'I'he expression alkali metal" as-is well known refers, or course, to the alkali elements, which as may-be seen byreterence for example to page 28 01' Part II or Outlines of Inorganic Chemistry" by F. A. Goocl and C. 1'. Walker, The MacMillan Company, N. Y. 1911, consist of the two common ones, the salts of. which are invery extensive commercial use, namely, sodium and potassium, together with the three less common ones, namely, lithium, rubidium and caesium. J I

Where the word pigment or sludge is used in the claims, it is meant to designate the precipitate' produced according to the process hereu in; and where in the claims it is stated that a removed, by suitable means such for example as described in the above referred to, as well as other prior Rafton patents.

Where in the claims the pigment or sludge is referred to as being of very fine particle size or slow settling, or as having particles of very fine size or slow settling, it is meant that the particlesof said pigment or sludge are of very fine size or slow settling in comparison with the particles of the standard-.of-reference prior art pigment produced from similar raw materials and under similar conditions of reaction, excepting only that the reaction temperature was substantially above 70 C. Where in the claims the term very fine size is employed, it is meant to include such fineness of size as that in which the particles approach or are in the colloidal contemperatures contemplated and that the operation referred to may be conducted either at the specified temperature or at a lower temperature down to substantially the freezing point of the mix.

Where in the claims reference is made to conducting an operation down to and not lower than substantially the freezing point of the mix; it is to be understood that the operation may be c nducted at a temperature just above the point at which the mix freezes, or at any temperature thereabove up to' and including the temperature limitation set out in the claims.

We use the term lime containing magnesia in its commonly accepted meaning which covers limes containing magnesia down to those which contain commercially unimportant proportions of magnesia, which latter limes are in contradistinction ordinarily termed high calcium limes or more briefly calcium limes.

Where in the claims we use the expression containing a carbonate group 'in referring to an alkali metal salt, we intend it shall include within its scope bicarbonates as well as carbonates of the alkali metals.

While there has been described in detail the preferred embodiments of the invention, it is to be understood that the details of procedure, the proportions of ingredients, and the arrangement of steps may be widely varied without departing from the spirit ofthe invention or the scope of the subjoir ed claims.

We claim: 1. In the manufacture of pigment conjoint] with alkali metal hydroxide by the reaction of previously slaked lime containing magnesia with a causticizable alkali metal salt in the presence of water, the improvement which consists in producing the pigment in the form of slow settling particles of very fine size in conjunction with alkali metal hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon said causticizable salt with said previously slaked lime containing magnesia at a temperature up to and not exceeding 70 C.,

and maintaining the reaction mix at a temperature up to and not exceeding 70 C. during at least a substantial part .of the reaction.

2. In the manufacture of. pigment conjointly with alkali metal hydroxide by the reaction in the presence of water of previously produced calcium hydroxide magnesium hydroxide with a causticizable alkali metal salt containing a carbonate group, the improvement which consists in producing the pigment in the form of slow settling particles'of very fine size in conjunction with alkali metal hydroxide in the presence of water by a modification of the causticizing operawith alkali metal hydroxide by the reaction of previously slaked lime containing magnesia with an alkali metal carbonate in the presence of water, the improvement which consists in producing the pigment in the form of slow settling particles of very fine size in conjunction with alkali metal hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon said alkali metal car bonate with said previously slaked lime. containing magnesia at a temperature up to and not exceeding 70 C., and maintaining the reaction mix at a temperature up to and not exceeding 70 C.

during at least such substantial part of the reaction as is'suflicient to insure the production of said pigment in said form of slow settling particles of very fine size, said lime containing magnesia having been previously slaked with an amount of water at least as great as that 'required to yield a thick paste.

4. In the manufacture of pigment conjointly with sodium hydroxide by the reaction of previously slaked lime containing magnesia with sodium carbonate in the presence of water, the improvement which consists in producing the pigment in the form of slow settling particles of very fine size in conjunction with sodium hydroxide in the presence of water by amodification of. the causticizing operation comprising reacting upon said sodium carbonate with said previously slaked lime containing magnesia at a temperature up to and not exceeding 70 C., and maintaining the reaction mix at a temperature up to and not exceeding 70 C. during at least a substantial part of. the reaction, reactive contact of said sodium carbonate with said previously slaked lime containing magnesia being effected by agitation.

5. In the manufacture of pigment conjointlying at least such substantial part of the reaction, I at a pressure not above substantially atmospheric.

6. In the manufacture of pigment conjointly with alkali metal hydroxide by the reaction in the presence of water of .previously slaked lime containing magnesia with a causticizable alkali metal salt containing a carbonate group, the improvement which consists in producing the pigment in the form of slow settling particles of very flne size in conjunction with alkali metal hydroxide in the presence of water by a modiilcation of the causticizing operation comprising reacting upon said causticizable salt with said previously slaked lime containing magnesia at a temperature up to and not exceeding 60 C., and maintainingthe reaction mix at a temperature up to and not exceeding 60 C. during at least such part of the reaction as is sumcient to insure the production of said pigment in said form of slow settling particles of very fine size.

7. In the manufacture of pigment conjointly with sodium hydroxide by the reaction of previously slaked lime containing magnesia with sodium carbonate in the presence of water, the improvement which consists in producing the pigment in the form of slow settling particles of very flne size in conjunction with sodium hydroxide in the presence of water by a modification of the 'causticizing operation compriaingreacting upon said sodium carbonate with said previously slaked lime containing magnesia at a temperature up to and not exceeding 50 C., and maintaining the reaction mix at a temperature up to and not exceeding 50' C. during at least such substantial part of the reaction as is sufllcient to insure the production of ,said pigment in said form of slow settlingparticles of very fine size, reactive contact of said sodium carbonate with said previously slaked lime containing magnesia being eifected by agitation, said, lime containing magnesia having been previously slaked with an amount of water at least as great as that required to yield a thick paste.

- 8. In the manufacture of pigment conjointly with sodium hydroxide by the reaction oi previously slaked lime containing magnesia with sond maintaining the reaction mix at a temperature up to and not exceeding 40 C. during at least a substantial part of the reaction, reactive contact of said sodium carbonate with said previously slaked lime containing magnesia being effected by agitation.

9. In the manufacture of pigment conjointly with alkali metal hydroxide by the reaction of previously slaked lime containing magnesia with a causticizable alkali metal salt in the presence of water, the improvement which consists in produc'ng the pigment in the form of slow settling particles of very fine size in conjunction with lkali metal hydroxide in the presence of water y a modification of the causticizing operation comprising reacting upon said causticizable salt with said previously slaked lime containing mag- 'nesia at a temperatureupatofand not exceeding 708 C., maintaining the reacticn mixat a temperature up to and not exceeding 70 C. during at least a substantial part of the reaction, and subsequently raising the temperature of the reaction'mix above 70 C.

10. In the manufacture of pigment conjointly with alkali metal hydroxide by the reaction in the presence of water of previously slaked lime containing magnesia with a caustlcizable alkali metal salt containing a carbonate group, the improvement which consists in producing the pigment in the form of slow settling Particles of very fine size in conjunction with alkali metal hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon said causticiaabie salt with said previously slaked lime containing (magnesia at a temperature up to and not exceeding 85 0.. mantaining the reaction mix at a temperature up to and not exceeding 65 C.'during at least such substantial part of the reaction as is sufficient to insure the production of said pigment in said form of slow settling particles of very fine size at a pressure not above substantially atmospheric, reactive contact of said causticizable salt with said previously slaked .lirne containing. magnesia being eflected by agitation, and subsequently raising the temperature of the reaction mix above 70 C.

11. In the manufacture ofpigment conjointly with alkali metal hydroxide by the reaction in the presence of water of previously slaked lime containing magnesia with a causticizable alkali metal saltcontaining a carbonate group, the improvement which consists in producing the pigment in the form of slow settling particles of very fine size in conjunction with alkali metal hydroxldein the presence of water by a modification of the causticizing operation comprising reacting upon said causticizable salt with said previously slaked lime containing magnesia at a temperature up to and not exceeding 60 C., maintaining the reaction mix at a temperature up to'and not exceeding 60 C. during at least very fine size in conjunction with alkali metal hydroxide. in the presence of water bya modiil cation of the causticizing operation comprisingv reacting upon said causticizable salt with said previously slakedlime containing magnesia at a temperature up to and not exceeding 50 C.,

maintaining the reaction mix at a temperature up to and notexceeding 50 C. during at least such substantial part of the reaction as is sumcientto insure the production of said pigment in said form of slow settling particles of very fine size, and subsequently raising the temperature of the reaction mix above 70 C.

13. In the manufacture of pigment conjointly with sodium hydroxide by the reaction of previousiy slaked lime containing magnesia with so- .dium carbonate in the presence of water, the im provement which consists in producing the pig ment in the term of slow settling particles 01175 very fine size in conjunction with sodium hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon said sodium carbonate with said previously slaked lime containing magnesia at a temperature up to and not exceeding 70 C., maintaining the reaction mix at a temperature up to and not exceeding 70 C. during at least a substantial part of the reaction, reactive contact of said sodium carbonate with said previously slaked lime containing magnesia being efiected by agitation, and subsequently raising the temperature of the reaction mix above 70 C., said lime containing magnesia having been previously slaked with an amount of water at least as great as that required to yield a thick paste.

14. In the manufacture of pigment conjointly with sodium hydroxide by the reaction of previous'ly slaked lime containing magnesia with sodium carbonate in the presence of water, the improvement which consists in producing the pigment in the form of slow settling particles of very fine size in conjunction with sodium hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon said sodium carbonate with said previously slaked lime containing magnesia at a temperature up to and not exceeding 40 C., maintaining the reaction mix at a temperature up to and not exceeding 40 C. during at least a substantial part of the reaction, reactive contact of said sodium carbonate with said previously slaked lime containing magnesia being effected by agitation, and subsequently raising the temperature of the reaction mix above 70 C.

15. The improvement in causticizing practice which consists in producing slow settling pigment of very fine particle size comprising calcium carbonate magnesium hydroxide conjointly with alkali metal hydroxide in the presence of water, by effecting causticizing of a causticizable alkali metal salt containing a carbonate group with previously slaked lime containing magnesia in the presence of water at a temperature down to and not lower than substantially the freezing point of the mix and not exceeding the highest at which the gelling phenomenon can occur, with gel formation, and thereafter dispersing the gel formed.

16. In a causticizing process, wherein a plurality of variable operating factors, including temperature, agitation, concentration, and proportion of causticizing agent added at the start of the reaction, are involved, the improvement which consists in producing slow settling pigment of very fine particle size comprising calcium carbonate magnesium hydroxide conjointly with alkali metal hydroxide in the presence'of water, by effecting causticizing of a causticizable alkali metal salt containing a carbonate group with previously slaked lime containing magnesia in the presence of water at a temperature down to and not lower than substantially the freezing point of the mix and not exceeding the highest at which the gelling phenomenon can occur, which temperature, without adjusting any other of said plurality of variable factors, would otherwise cause gelling, and substantially preventing such gelling from taking place by adjusting at least one other of said plurality of variable factors.

17. In a method of producing slow settling pigment of very fine particle size comprising calcium carbonate magnesium hydroxide conjointly with alkali metal hydroxide in the presence of water in a causticizing reaction between previously slaked lime containing magnesia and a causticizable alkali metal salt containing a carbonate group conducted at a temperature down to and not lower than substantially the freezing point of the mix and not exceeding the highest at which the gelling phenomenon can occur, wherein agitation of the reaction mix is efiected, and wherein a gel would otherwise be formed if vigorous agitation were employed during at least the early stage of the reaction, the step comprising substantially avoiding gelling by limiting the vigor of the agitation during at least the early stage of the reaction.

18. In a method of producing slow settling'pigment of very fine particle size comprising calcium carbonate magnesium hydroxide conjointly with alkali metal hydroxide in the presence of water in a. causticizing reaction between previously slaked lime containing magnesia and a causticizable alkali metal salt containing a carbonate group conducted at a temperature down to and not lower than substantially the freezing point of the mix and not exceeding the highest at which the gelling phenomenon can occur, wherein a gel would otherwise be formed if the concentration. of the calcium oxide in the causticizing agent calculated in the form originally used were substantially .7 pound or more per gallon of total volume of reaction mix, the step comprising substantially avoiding gelling by erecting the reaction at a concentration less than substantially .7 pound calcium oxide in the causticizing agent calculated in the form originally used per gallon of total volume of reaction mix.

19. In a method of producing slow settling pigment of very fine particle size comprising calcium carbonate magnesium hydroxide conjointly with alkali metal hydroxide in the presence of water in a causticizing reaction between previously slaked lime containing magnesia and a causticizable alkali metal salt containing a carbonate group conducted at a temperature down to and not lower than substantially the freezing point of the mix and not exceeding the highest at which the gelling phenomenon can occur, wherein a gel would otherwise be formed if the total amount of causticizing agent were added at the start, the steps comprising substantially avoiding gelling by adding less than the total amount of causticizing agent at the start, and subsequently completing the addition of said ingredient.

20.In the manufacture of calcium carbonate magnesium hydroxide conjointly with sodium hydroxide, the improvement which consists in producing the calcium carbonate magnesium hydroxide in the form of slow settling particles of very fine size in conjunction with sodium hydroxide in the presence of water by a modification of the causticizing operation comprising slaking lime containing magnesia with water, reacting the slaked lime containing magnesia so produced with sodium carbonate in the presence of water at a. temperature up to and not exceeding 70 C., at least a substantial part of the reaction being conducted at a temperature up to and not exceeding 70 C., relative contact of said sodium carbonate and said previously slaked lime containing magnesia being eifected by agitation, and separating the resultant solution of sodium hydroxide from the precipitate formed.

21. In the manufacture of calcium carbonate magnesium hydroxide conjointly with sodium hydroxide, the improvement which consists in producing the calcium carbonate magnesium hydrox--- ide in the form of slow settling particles of very fine size in conjunction with sodium hydroxide in the presence of water by a modification of the causticizing operation comprising slaking lime containing magnesia in a substantial excess of water, employing vigorous agitation, reacting the slaked lime containing magnesia so produced with sodium carbonate in the presence 01' water at a temperature up to and not exceeding 70 C., maintaining the reaction mix during at least a substantial part of said reaction at a temperature up to and not exceeding 70 C., separating the resultant solution from the precipitate formed,

reacting the precipitate with further quantity of sodium carbonate in the presence of water, and then separating the resultant slow settling very fine particle size calcium carbonate magnesium hydroxide which is substantially free from calcium hydroxide from the associated liquor.

22. In the manufacture of pigment conjointly with alkali metal hydroxide, the improvement which consists in producing the pigment in substantially colloidal form in conjunction with alkali metal hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon a causticizable alkali metal salt with previously slaked lime containing magnesia in the presence of water ata temperature up to and not exceeding 70 C., and maintaining the reaction mix at a temperature up to and not exceeding 70 C. during at least a substantial part of the reaction, the concentration of calcium oxide in the causticizing agent calculated in the form originally used to total volume of reaction mix being less than substantially .7 lb. to the gallon.

23. In the manufacture of calcium carbonatemagnesium hydroxide conjointly with sodium hydroxide, the improvement which consists in producing the calcium carbonate magnesium hydroxide in substantially colloidal form in conjunction with sodium hydroxide in the presence or water by a modification of the causticizing operation comprising reacting upon sodium carbonate with previously slaked lime containing magnesia in the presence of water at a temperature up to and not exceeding 50 C., and maintaining the reaction mix at a temperature up to and not exceeding 50 C. during at least such substantial part of the reaction as is sufi'lcient to insure the production of said calcium carbonate magnesium hydroxide in said substantially colloidal form, the concentration of calcium oxide in the causticizing agent calculated in the form originally used to total volume of reaction mix being less than substantially .7 1b. to the gallon.

24. In the manufacture of calcium carbonate magnesium hydroxide conjointly with sodium hydroxide, the improvement which consists in producing the calcium carbonate magnesium hydroxide in substantially colloidal form in con- Junction with sodium hydroxide in the presence or water by a modification 01 the causticizing previously slaked lime containing magnesia in operation comprising .reacting upon sodium oarbonate with previously slaked lime containing magnesia in the presence 01' water at a temperature up to and not exceeding 40 C., and maintaining the reaction mix at a temperature up to 5 and not exceeding 40 C. during at least a substantial part of the reaction, the concentration of calcium oxide in the causticizing agent calculated in the form originally used to total volume of reaction mix being less than substantially .7 1b. 10 to the gallon.

25. In the manufacture of calcium carbonate magnesium hydroxide conjointly with sodium hydroxide, the improvement which consists in producing the calcium carbonate magnesium hydrox- 15 ide in substantially colloidal form in conjunction with sodium hydroxide in the presence of water by a modification of the causticizing operation comprising reacting upon sodium carbonate with the presence of water at a temperature up to and not exceeding C., and maintaining the reaction mix at a temperature up to and not ex-. ceeding 60 C. during at least a substantial part of the reaction, the concentration of calcium oxide 25 in the causticizing agent calculated in the form originally used to total volume 01 reaction mix being less than substantially .7 lb. to the gallon.

26. A causticizing sludge thereby characterized that it is the substantially water insoluble reaction product of a previously slaked lime containing magnesia and a causticizable alkali metal salt containing a carbonate group, that substantially all the particles thereof are slow settling and of very fine size as precipitated, and that it consists of the substantially water insoluble product of reaction of said reagents carried out, in the presence of water, at least in substantial part at a temperature up to and not exceeding C., the reaction mix having been maintained at a temperature up to and not exceeding 70 C. during at least a substantial part of the reaction.

27. A causticizing sludge thereby characterized that it is the substantially water insolublereaction product of a previously slaked lime containing magnesia and sodium carbonate, that substantially all the particles thereof are slow settling and of very fine sizeas precipitated, and that it consists oi the substantially water insoluble product of reaction of said reagents carried out, in the presence of water, at least in substantial part at a temperature up to and not exceeding 40 C. with agitation, the reaction mix having been maintained at a temperature .up to and not 55 exceeding 40 C. during at least a substantial part of the reaction, said lime containingmagnesia having been previously slaked with an amount of water at least as great as that required to yield a thick paste.

ARTHUR MINARD BROOKS. HAROLD ROBERT RAF'I'ON.

CERTIFICATE OF CORRECTION.

Patent nQ..2 ,oee,oee. 1 v g December29,195 6.

ARTHUR MINARD BROOKS, ET At. g

It is hereby certified that error appears in the printedspecifi'cation of the above numbered patent requiring correction as follows; Page 2, first .column, line 28; for the word "bud" read mud; and second column, line 24,- for "limits" readli mes; page '7, second column, -line 37; page 8, first f column, line 36, and second column, lines 20 and .64; and page 10, first column, line 49,- claims 3, "7, 10, 12 and 2-3 respectively, strike out'the word "substantial" page 8, first column, line 1, claim 5,"strike out "such" and insert instead the articlea; and-second column, line 25'," claim 10, after- "size" insert a comma; page 9, second column, line 50, claim 18,

for "erecting" read effecting; line 67, a claim 20,] for "relative" read reactive; and that the said Letters Patent should .be readv with these corrections therein that the same may conform to the record of the "case in the Patent Office. V I

Signed and sealed this 9th day of March, A. D. 1937.

3 Henry Van Arsdale (Seal) Acting Commissioner of Patents. 

